The need for spill-proof cups, as used by infants and the infirm, is well known. These cups are liquid-tight, preferably also air-tight, and are designed not to leak when the cup is held in a tilted or overturned position by a child, or when the cup falls on its side or even turns over. Preferably, the cup should also resist spillage when shaken or swung, as happens when children carry it around carelessly.
There are various known designs serving this purpose. A first design requires some deliberate action to be taken to seal and/or open the cups and such cups suffer from the obvious disadvantage that an infant cannot be relied on to operate the closure. A second design includes a pressure operated valve that is intended to open automatically in response to a reduced pressure in the spout, and to reseal when the suction is removed. Such valves suffer from the general problem that they cannot distinguish between high pressure within the container and low pressure in the spout. Therefore these valves are either not efficient in blocking leaks, or else they offer an undesirable level of resistance to suction.
A still further problem with cups having pressure operated valves is that they cannot safely be used with carbonated or hot beverages. In the latter case, when the cup is inverted the liquid heats the air in the ullage space and increases the pressure within the container because the outlet is already covered by the liquid, which is then driven out.
To avoid the above disadvantages, the present invention uses a valve known as a self-sealing demand valve, the self-sealing referring to the fact that the pressure inside the container acts to close the valve rather than to open it. Another advantage of such a valve is that it can be designed in a way that allows the valve to be opened by a very low suction level
A simple general way to implement a self-sealing demand valve is by constructing the valve such that the valve closure element moves against the direction of fluid flow when opening the valve. Examples of this kind of valve are to be found in U.S. Pat. Nos. 5,409,035, 3,493,011 and 6,554,023. The valve contains a diaphragm that is subjected on a first side to a fixed pressure, usually atmospheric, and acts on its second side on a valve closure element. The valve closure element is biased towards a closed position in which it seals against a valve seat. The pressure on the outlet side of the cup acts on the second side of the diaphragm so that, when suction is applied to the outlet, the diaphragm forces the valve closure element in a direction to lift the closure element off its seat and open the valve. In the absence of suction at the outlet, the biasing force of the valve closure element returns it to the closed position and keeps it closed, and any positive pressure at the valve inlet increases the closing force.
Valves of this nature have not yet been applied to mass-market spill-proof cups for use by infants because of certain requirements that need to be met. In particular, it is important for all the following criteria to be met, namely:                all sides of all components need to be easily accessible for thorough cleaning,        the diaphragm, when installed, needs to be exposed to outside atmospheric pressure but must not be accessible to accidental contact from outside the cup,        the number of separate components needs to be minimised and their forms must be designed so as to minimise manufacturing cost and to ease handling,        the disassembly for cleaning must be easy to perform and it should only be possible to reassemble the components in one way, and        for safety reason, components smaller than a specified minimum size must not be used.        
Packaging of a demand valve within a spill-proof cup also presents difficulties in that the volume between the valve and the spout of the cup needs to be kept to a minimum. This volume will, after drinking, remain filled with liquid and may subsequently run out or shake out. It has been proposed in the prior art to interpose a small orifice but this would make it more difficult to drink from the cup. It would also adversely affect the ease with which the valve components can be moulded and cleaned.
WO03/068036, which is believed to represent the closest prior art to the present invention, discloses a cap for closing a liquid beverage container for allowing the beverage to be sucked out of the container through a spout, while preventing spillage when no suction takes place. A demand valve is incorporated into the cap, the demand valve having an inlet port communicating with the interior of the container, a discharge port communicating with the spout and a control port communicating with the ambient atmosphere through a hole in the cap. The valve has a valve seat and a closure element controlling the flow from the inlet port to the discharge port, the closure element being urged by the pressure in the container in a direction to seal against the valve seat and being urged to move to an open position in dependence on the excess of the pressure in the control port over that in the discharge port. The movement of the closure element to open the valve is in the opposite direction to that of the flow of liquid through the valve. The demand valve is formed by two members that are mounted within the spout of the cap. The first member is rigid and defines the valve inlet port and the valve seat. The second member incorporates the valve closure element and a resilient membrane which serves as a pressure sensitive diaphragm, the second member sealing against the first member and around the hole in the cap.
The spill-proof cup of the latter patent specification could not be made to operate satisfactorily and has not been marketed. Because of a feature inherent in its design, the second member of the demand valve could not be made to seal reliably against the first member.